Image display devices such as cathode-ray tube (CRT) display devices, liquid crystal displays (LCDs), plasma display panels (PDPs), electroluminescence displays (ELDs), field emission displays (FEDs), touch panels, electronic paper, and tablet PCs are generally provided with an optical layered body for antireflection on the outermost surface thereof. Such an antireflection optical layered body reduces double image reflection and lowers the reflectivity by light diffusion and interference.
As one of the antireflection optical layered bodies, an antiglare film including an antiglare having a surface roughness on a transparent substrate surface is known. Such an antiglare film can diffuse external light by the surface roughness on the surface thereof to prevent a decrease in visibility caused by external light reflection and double image reflection.
Known examples of conventional antiglare films include a type in which the surface roughness is formed on the surface of the antiglare layer by agglomeration of particles such as cohesive silica; a type in which an organic filler is incorporated into a resin and the surface roughness is formed with the organic filler on the layer surface; and a type in which the surface roughness is transferred to the layer surface by lamination of a film having projections and depressions.
All of these conventional antiglare films are formed to achieve light diffusion and antiglare effects by the function of the pattern of the antiglare layer surface. The projections and the depressions on the antiglare layer surface must be enlarged in order to enhance the antiglare property. However, as the projections and depressions are enlarged, the haze value of a coating film increases, causing discoloration. This unfortunately results in a decrease in transmission image definition.
Additionally, for example, in the case where a conventional antiglare film is used in a liquid crystal display device, when transmitted light from the back (for example, a back light) passes through the antiglare film having a surface roughness, the effect of such surface roughness of the film surface unfortunately produces “scintillation” that disrupts displayed pixels (scintillation appears as flashing “flickering” to the human eye, caused by the difference in luminance).
In order to prevent the occurrence of scintillation, there is a method, for example, in which internal diffusion is created by particles whose refractive index is different from that of a binder resin constituting the antiglare film.
In the case of preventing scintillation by the internal diffusion, the refractive index difference between the particles and the binder resin constituting the antiglare film must be increased, or the amount of particles must be increased.
However, when such a method for preventing scintillation is applied, there will be an interface between the binder resin and the particles, and consequently, reflection will occur unfortunately at the interface according to the refractive index difference between the binder resin and the particles, thus decreasing the contrast of the resulting image.
As a solution to the above problem, for example, there is a known method in which multi-layer particles having a refractive index that gradually changes from the core to the surface layer, and particles having a graded refractive index are used (for example, see Patent Literature 1). Additionally, for example, there is another known method in which particles having, at the particle surface, a layer with a thickness of about 100 nm and a refractive index is intermediate between the refractive indices between the binder resin and the particles are used in order to decrease the reflection at the interface between the binder and the particles (for example, see Patent Literature 2).
However, these particles are very expensive. In addition, due to production methods thereof, it is difficult to control the particle size and to increase the refractive index difference between the particle surface layer and the core. Thus, it has been difficult to increase internal diffusion.
Attempts to alleviate problems relating to scintillation, contrast, and discoloration have been made by using two types of particles having different sizes or two types of particles having different refractive indices at a specific mixing ratio (for example, see Patent Literatures 3, 4, 5, 6, 7, and 8). However, considerations are not given in a comprehensive manner in terms of particle sizes of particles that contribute to the surface roughness on the surface and particles that contribute to internal diffusion; mixing ratio; refractive indices; and the like. Thus, in reality, an antiglare film having excellent antiglare property, contrast, and scintillation is yet to be achieved.